1. Field of the Invention
The present invention relates to the field of crystalline materials processing and, more particularly, to the removal of material from the surfaces of crystalline structures by etching.
2. Art Background
Of crucial importance to the semiconductor industry is the ability to produce very precise and very minute electronic circuits on the surfaces of semiconductor materials, most commonly crystalline silicon. In order to produce high quality semiconductors, the surface of such crystalline materials must be properly prepared.
Etching is a method of processing crystalline materials in which acids are used to dissolve away desired portions of a crystalline surface. A mask is placed over the surface of the crystal, leaving exposed the areas from which material is to be removed, and the crystal is immersed in an etchant. Commonly used etchants include potassium Hydronide (KOH), Hydrofloric Acid (HF), NaOH (for Silicon) and sulfuric acid (H.sub.2 SO.sub.4). By controlling the time that the crystal is immersed in the etchant very precise amounts of material can be removed. Unfortunately, however, some important materials used in semiconductor manufacturing, such as sapphire, are inherently resistant to etching.
Sapphire, a crystalline form of Al.sub.2 O.sub.3 having a density of 3.98, a melting point of 2040.degree. C. and a Mohs hardness of 9.0, is becoming increasingly important as a substrate for thin film components and integrated circuits because of its inherent radiation resistance, smooth surface finish and high dielectric constant (480 KV/cm). Silicon on Sapphire (SOS) semiconductors, originally developed for critical military applications, are being increasingly used wherever high speed, low power dissipation and high radiation resistance are desired.
Despite having many desirable properties, the use of sapphire in semiconductors is limited by its resistance to conventional etchants. As shown in Table 1, on page 435 of Thin Film Processes by Vossen & Kern (Academic Press 1978), conventional etchants, including phosphoric acid, are ineffective for etching sapphire.
The present invention, however, discloses a method with which phosphoric acid, previously thought to be ineffective for etching sapphire, can be used to successfully etch sapphire. The method may also be adapted to etch other materials that have previously shown a high resistance to conventional etchants.